This invention relates to a method for manufacturing a semiconductor device especially directed to formation of a trench.
To maximize the capacity obtained from a limited area of a semiconductor memory device, for example, a trench capacitor is used, and it is necessary to make a deep trench.
Such a deep trench is usually made by an etching process. As the hard mask for this etching, a CVD SiO.sub.2 film obtained by decomposing TEOS (tetraethoxysilane, Si(OC.sub.2 H.sub.5).sub.4) by low-pressure CVD (LP-CVD) has often been used conventionally.
However, this film involves a problem that it is difficult to remove it evenly when it is etched-off by dry etching or other method after a trench is made. This invites the problem that a difference in level is formed on a silicon oxide film after the dry etching for removing the CVD SiO.sub.2 film has been performed and this level difference adversely affects subsequent steps of the manufacturing process. Moreover, in case of TEOS, there is another problem that the CVD SiO.sub.2 film cannot be etched off soon after etching for forming trench.
On the other hand, there are doped oxides like BSG, BPSG, PSG as materials that have been treated similarly to insulating materials of the group of CVD-SiO.sub.2 referred to above. These BSG, BPSG and PSG exhibit a higher etching speed during wet etching as compared with a TEOS film, for example, and they are advantageous because they can be readily removed after being used as a hard mask.
FIGS. 1A through 1D show cross-sectional views in different steps in a process of making a trench by using BSG as the doped oxide.
First formed on a silicon semiconductor substrate 11 is a BSG film by CVD, for example. Further coated on the BSG film is a resist 13 by spin coating, for example (FIG. 1A).
After that, exposure and development are conducted by using an exposure mask corresponding to a desired pattern, and the resist 13 is patterned (FIG. 1B).
Then, by using the patterned resist 13' as an etching mask, the BSG film 12 is etched (FIG. 1C).
Subsequently, the resist 13' is removed, and by using the etched BSG film 12' as a hard mask, silicon 11 is etched to form a deep trench (FIG. 1D).
After the trench is made, the BSG film 12' is removed.
However, in the case where a doped oxide like BSG, BPSG, PSG, or the like, as the hard mask material, the etched shape is more liable to vary than using other CVD SiO.sub.2 film as the hard mask.
It is believed that this problem occurs because, due to the natures of BSG, BPSG and PSG readily absorbing moisture, the moisture contained in the BSG, BPSG or PSG film is released into the chamber atmosphere during processing, and this invites fluctuation of the etching characteristics.
Since the quantity of moisture absorbed in BSG, BPSG or PSG varies with the atmosphere in which the sample is exposed, it is impossible to prevent that the processing characteristics delicately change with the exposing condition and other factors.
Therefore, when a BSG, BPSG or PSG film is used, there is a problem that the etching conditions have to be changed unlike the cases using a SiO.sub.2 film.